Combined Impact of Denticity and Orientation on Molecular-Scale Charge Transport

Reducing the dimensions of electronic devices to the nanoscale is an important objective with significant scientific and technical challenges. In molecule-based approaches, the orientation of the molecule and coordination to electrodes (denticity) can dramatically affect the electrical properties of the junction. Typically, higher conductance is associated with shorter transport distances and stronger molecule-electrode coupling; however, this is not always the case, as highlighted in this study. We focused on 7,7,8,8-tetracyanoquinodimethane (TCNQ) and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F(4)TCNQ) molecules and have used the scanning tunneling microscopy break junction (STM-BJ) method to measure the electrical conductance of single molecules bridged between gold electrodes with different molecular orientations and with varying denticities. In conjunction with the experiments, density functional theory (DFT) and nonequilibrium Green's function (NEGF) calculations were performed to determine the conductance of four distinct molecular configurations. The calculated conductances show how different configurations and denticities influence the molecular orbital offsets with respect to the Fermi level and provide assignments for the experimental results. Surprisingly, lower denticity results in higher conductance, with the highest predicted molecular conductance being 0.6 G(0), which is explained by the influence of molecule-electrode coupling on the energy of molecular orbitals relative to the Fermi level. These results highlight the importance of molecular geometry and binding configuration of the molecule to the electrode. Consequently, our findings have profound ramifications for applications in which orbital alignment is critical to the efficiency of charge transport, such as in dye sensitized solar cells, molecular switches, and sensors.